An effort will be made to produce haploid mouse cells by combining haploid blastomeres with normal diploid blastomeres to make a haplo-diploid mosaic mouse embryo which on reimplantation into a recipient female might develop far enough to produce semi-differentiated haploid cells that could then be cultured in vitro. Haploid blastomeres will be produced by surgical removal of one pronucleus from a recently fertilized egg. Such eggs with a single haploid nucleus will cleave an produce haploid blastomeres but, extensive development does not occur. Combining such haploid cells with diploid blastomeres may enhance the development of the haploid cells. The capacity of cells taken from advanced embryonic stages of the mouse to participate in early stages of embryogenesis will be tested by implanting such cells into the blastocyst cavity of a mouse embryo. Such cells may be incorporated into the inner cell mass and thus contribute to a wide variety of cells in the developed embryo. By such procedures it should be possible to test the stability of the differentiated state of cells in embryos as well as to test the regulative capacity of the early mammalian embryo. BIBLIOGRAPHIC REFERENCE: Hammond, G. L., J. Lewis, B. Nadal-Ginard, and C. L. Markert. 1975. Shifts in myocardial LDH isozyme patterns during chronic cardiac ischemia in man. Symp. on Coronary Artery Disease. Texas Heart Institute. In press.